This invention relates to presses of the type wherein two pressure platens are each supported by a support construction so that the forces of each of the press platens are transmitted to their corresponding support construction and in which means are provided to regulate spacing of the press platens including pressure elements positioned between the platens and respective support construction in general, and more particularly, to a rapid pressure release device for such a press which prevents excessive pressures building up therein.
A press of this general nature is disclosed in U.S. Pat. No. 3,851,658, the disclosure of which is hereby incorporated by reference. Means for maintaining constant spacing in such a press are described in detail in U.S. Pat. No. 3,881,852, which is also hereby incorporated by reference. The press disclosed in these references is a continuous press for the manufacture of wood, chipped wood or the like, in which the material to be treated is moved continuously between two flexible endless conveyor belt spans. Between the belt span forming the press zone and the press platens, a plurality of rotatively unpowered endless loops of roller chains are used, these roller chains are used, these roller chains being packed transversely together to form a bed interposed between the platens and the seal strip conveyor belts. The press platens in turn are positioned between support constructions comprising beams, each position transversely to the movement of the belts with portions of such beams extending longitudinally above and below the press platens for the length of a press.
Hydraulic pressure cushions or hydraulic cylinders are interposed between the beam and pressure platen to control pressure and maintain the proper spacing. In the second of the above-mentioned references, means for controlling this spacing in the form of a guiding valve arranged so that when the distance between the press platens is at a pre-determined spacing, the valve is inactive and is closed. A pressure pump coupled through the valve causes an increase in the pressure in the pressure elements until the press platens begin to move together changing their position. At that point, the guide valve opens to allow a bleed off of hydraulic fluid relieving the pressure so that the press platen moves back in a direction to cause it again to be at the pre-determined distance, where upon reaching the pre-determined distance, the valve becomes inactive. Through this arrangement, a dynamic balance about the pre-determined distance between the platens is achieved. Such a design works quite well. However, if the material being compressed for some reason present to the press a greater resistance to compression than is normal, the pressure in the pressure elements increases.
As is well known, presses of this nature are used for the continuous or discontinuous production of panel materials. With many materials of this nature, during formation of the panel, a chemical process occurs with which is associated the development of gases, or in any case a tendency for an increase in volume. For example, such occurs when producing panels of a foam substance. In such processes production speed can be increased through the use of heat. However, at the same time, the process generates heat. Thus, the situation can arise where the temperature within the mass of the material rises too fast, accelerating the process and generating still more heat and further acceleration of the process. Such may lead to an explosive rise in pressure which may burst the entire press mechanism.
In view of the possibility of such occurring, the need for a safety device which can rapidly relieve pressure if the process gets out of hand and the pressure rises abruptly to prevent further pressure increases becomes evident. Such a device is needed if safe operation at high production speeds is to take place.